1. Field of the Invention
This invention relates to a tone display method that overlaps and displays multiple subfields obtained by weighting dynamic images having tones in the time domain in a DLP (digital light processing) image display device using one reflection-type device or other image display device that displays color images by the time-division output of color signals.
2. Brief Description of the Prior Art
In an image display device using the digital display method, such as a veneer DLP image display device using one reflection-type device, a method such as disclosed in U.S. Pat. No. 5,448,314 can be used to display color signals.
FIG. 7 shows an example of the configuration of an image display device. FIG. 8 shows an example of the configuration of the color wheel (2) shown in FIG. 7. Color wheel (2) consists of color filters of green G, red R and blue B with each central angle set at 60.degree.. In order to reduce color separation, the color wheel can be set to have, for example, three rotations for one field.
In FIG. 7, 8-bit digital color signals of RGB are input to time division multiplex circuit (5). Each of the input color signals of RGB is time-.base compressed to the 1/3 period, time-division multiplexed, and output. FIG. 9 shows the pattern obtained by time-division multiplexing the period of one field. A DMD (digital micromirror device) (3) is controlled by the output signal. Also, the white light emitted from lamp (1) used as the light source reaches the DMD (3) after passing through color wheel (2) used as the color extraction device. In this case, the light of colors G, R, B, corresponding to the periods shown in FIG. 9 and obtained after the white light is transmitted through the color wheel (2) and reaches DMD (3). In FIG. 9, the periods divided corresponding to the colors G, R, B are referred to as segments. In the following explanation and figures, the segments corresponding to colors G, R, B are represented by s, which means segment, and a number counted from the head or start of one field. In this case, color wheel (2) rotates clockwise with the light emitted from lamp (1) set at the position of 0.degree. (12 o'clock). The light in colors G, R, B is reflected by DMD (3) controlled by the corresponding periods and signals of G, R, B. The output optical signals of G, R, B are irradiated sequentially on screen (4) and are sensed as color images.
Each segment of the signals of G, R, B consists of multiple time-base divided subfields used for tone display. As an example of the subfield period, the subfield configuration of the first segment of G in the case of displaying 8 bits, that is 256 tones, is shown in the low-order portion of FIG. 9. FIG. 10 shows the subfield configurations of the other segments of G. FIG. 10 only shows the case of G. However, the patterns are the same for R and B. Reference is made to the tone display method disclosed in Japanese patent application to Kokai, No. Hei 9 [1997]-34399, etc., for the subfield configuration disclosed in this example.
In FIG. 10, the portion encircled by a rectangle is a subfield, and the length of the rectangle is the subfield period (time length of the subfield). In the following explanation and figures, the nth subfield is represented by SFn. The time length of each subfield is defined as the weight corresponding to the brightness of one color when only the segment concerned is turned on. In the case of the configuration shown in FIG. 7, the weight corresponds to the time when the mirror of DMD is on (lit) or to the number of lighting pulses during the time length. In the following figures, the value of the weight will be displayed below the rectangle that represents each subfield.
The process of selecting the appropriate subfield to be turned on (to be lit up) corresponding to the tone to be displayed will now be explained.
In a conventional example, one field is divided into 34 subfields for one color. Among the 8 bits, the 5 high-order bits are displayed by continuous time-width modulation using 31 subfields, that is, subfields SF4-SF34 having a weight of "8". The 3 low-order bits are displayed on the binary base using 3 subfelds SF1-SF3 having weights of "1", "2" and "4", respectively. In other words, for the subfields used to display the 5 high-order bits, whenever the tone is increased by 8, that is, whenever the 5 high-order bits have a carry of one, the number of lit subfields is increased one at a time in the sequence from SF4-SF34. Each pixel can display the tones by lighting up the subfields as described above. Since the line of vision is almost fixed on a stationary image, the image quality will not deteriorate by adding subfields for each pixel.
In the aforementioned conventional tone display method using subfields, pseudocontour noise is observed for dynamic images which deteriorates the image quality. Occurrence of pseudocontour of dynamic images is described in the reference ("Studies on Improving Dynamic Image Quality of PDP in Subfield Display" (Japanese Title), which is synonymous with "Consideration on Improving Motion Picture Quality of PDP with use of a Sub-Field Method" (English Title), IEICEJ Technical Report, EID 97-54 (1997-10), pp. 43-48). Since the line of vision moves following the track of a dynamic image, pseudocontour occurs because the position where the eyes process time integration changes in the space following the movement of the line of vision. In other words, when the line of vision moves at a speed to cover multiple pixels during the display period of one field, subfields are added not only in one pixel but over multiple pixels. As a result, the original image cannot be obtained, and the image quality is deteriorated.
FIG. 12 is a diagram explaining a tone display method, which displays 256 tones using n subfields. In this case, the aforementioned problem becomes extremely serious. Pixels A and B are arranged adjacent to each other. Pixel A displays 127 tones with subfields SF1-SFm (m=n/2) turned on and subfields SF (m+1)-SFn turned off. Pixel B displays 128 tones with subfields SF1-SFM turned off and subfields SF (m+1)-SFn turned on. In FIG. 12, the pixels are arranged in the vertical direction, while the subfields are arranged in the horizontal direction. In other words, the vertical direction in FIG. 12 indicates the spatial movement of the vision spot, while the horizontal direction indicates the time movement of the vision spot. In this case, when the vision spot does not move from pixel A (arrow c), the integral value of one field of pixel A becomes 127 tones as displayed. However, when the vision spot moves from pixel A to pixel B at a speed of 2 pixels per field (arrow a), both of the integral values of pixels A and B become 255 tones. When the vision spot moves from pixel B to pixel A at a speed of 2 pixels per field (arrow b), both of the integral values of pixels A and B become 0 tone.
In order to measure the actual image quality deterioration in a quantitative manner, FIG. 11 shows a computer-simulated image observed when a moving lamp waveform is displayed using the tone display method with the subfield configuration shown in FIG. 10. The basic method of the simulation is described in the above-mentioned reference. In this example, subfields are divided into 6 segments. Each unit period of each subfield is appropriately set corresponding to the position in accordance with the aforementioned division in one field. In this simulation, the time integral level of the eyes is calculated when the vision spot moves to the right at a speed of 8 pixels during the display period of each field. In this case, the simulation results are for G. However, the same results can be obtained for R and B. In the following, the case of G will be explained as an example.
For the lamp waveform used in this case, the level (tone) moves up by one corresponding to a movement of one pixel to the right in the horizontal direction. Levels 0-255 correspond to the horizontal positions 0-255, respectively.
When the lamp waveform is input, the waveform observed by the eyes should be a straight oblique line. However, the waveform becomes that shown in FIG. 11 when the conventional tone display method with the subfield configuration shown in FIG. 10 is used. The main reason for such a problem is that relatively strong noise occurs at the positions where the tone is a multiple of 8, that is, at the positions where the subfields with a weight of 8 are freshly lit up. More specifically, relatively strong noise corresponding to eight levels is generated near the positions where the tone to be displayed changes from 8.times.n-1 to 8.times.n (n is an integer in the range of 1-31).
The reason for the aforementioned noise will be explained based on the case in which the tone to be displayed is increased by one from 39 to 40. When the tone to be displayed is 39, all of the subfields of Gs1 are turned on. When the tone is changed to 40, SF1-SF3 of Gs1 are turned off, and SF8 of Gs2 is turned on. The on/off switching of the subfields is performed across the segments. Also, the total weight of the subfields switched to the off state in Gs1 is "7", while the total weight of the subfields switched to the on state in Gs2 is "8". When on/off switching of the subfields is performed across the segments, the movement interval (period) is usually expanded. As a result, the moving range of the vision line is expanded, and noise tends to occur. In particular, when the subfields as the objects of the on/off switching have a large total weight, the noise becomes more significant. On the other hand, if on/off switching of the subfields is not performed across the segments, the movement interval (period) becomes narrow for a change within a segment. As a result, the moving range of the vision line is reduced, and the chance for the occurrence of noise is reduced. For example, when the tone to be displayed is in the range of 0-39, that is, when only the subfields in Gs1 are used, little noise occurs even when SF1-SF3 are changed to any of SF4-SF7. As shown in FIG. 11, almost no noise occurs in the portion corresponding to the tone range of 0-39.
In the aforementioned conventional tone display method using subfields, when a picture is observed following the movement of the image, a significant brightness difference occurs between the pixels that originally have little difference in brightness, leading to an unnatural feel. Consequently, pseudocontour image-quality deterioration results for an image of the human body or other image that changes gradually.
In order to solve the aforementioned problems in the conventional tone display method using subfields, the subfields are divided in a finer manner, and the weight of each subfield is reduced to a level close to the minimum weight ("1" in the tone display method shown in FIGS. 9 and 10). However, more subfields result in an increase in the memory capacity and, therefore, an increase in power consumption. When the subfields are divided on a very small scale, the cost will be increased significantly. Consequently, it is desired to develop a tone display method that can prevent the image quality deterioration of dynamic images and can avoid an increase in cost by preventing the increase in the number of subfields to the extent possible.